Automatic frequency control receiver



June 20, 1939. D. E. FOSTER AUTOMATIC FREQUENCY CONTROL RECEIVER- FiledJan.. 5, 1958 ATTORN EY Patented June 20, 1939 iJNiTED- smrps AUTOMATICFREQUENCY CONTROL RECEIVER Dudley E. Foster, South Orange, N. J.,assigner to Radio Corporation of America, a corporation of DelawareApplication January 5, 1938, Serial No. 183,419

8 Claims. f (Cl. Z50- 20) My present invention relates to kautomaticfrequency control (AFC) arrangements for radio receivers; and moreparticularly the invention relates to an improved discriminator networktherefor.

There has been disclosed in the past discriminator, or frequencyresponse, networks for AFC arrangements, wherein the discriminatorcomprises a pair of separate diodes arranged in opposed relation withrespect to an I.F.tuned input circuit. The input circuit was constructedand arranged to impress on the diodes an alternating current voltagewhose magnitude and phase depended upon the magnitude and sense of theI. E', energy frequency shift. Inrsuch a discriminator net- .'ork it hasthought necessary to employ independent cathodes for the two diodes.However, in the utilization of a discriminator network it is frequentlydesirable to ground the diode cathodes. For example, a double diode tubeof the type employing a common cathode could be employed. Again, aduo-diode-triode (or pentode) could be utilized thereby permitting thedouble diode section to be used for discrimination and audio detection,while the amplifier section could be used for audio amplification.

Accordingly, it may be stated to be one of the main objects of mypresent invention to pro-vide a frequency discriminator networkemploying a pair of diodes having a commoncathode, and the diode loadresistors developing D. C. voltages whose relation is determined by thefrequency deviation of signal energy impressed on the cominon inputcircuit of the network.

Another important vobject of this invention is to provide in adiscrminator network of the phase shift type, a pair of diode rectiersprovided with a common cathode; the rectiiiers developing two D. C.voltages employed for frequency correction in a receiver, and the sum ofthe voltages being used to provide a voltage for automatic volumecontrol (AVC),

Still another desirable object of the invention is to provide in adiscriminator network of the phase shift type, a tube having a cathodecommon to a pair of diode sections and an amplifier section; the diodesections producing two D. C. voltages for AFC and AVC functions, and theamplifier section being used to amplify the audio output of the diodesections.

Still other objects of my invention are to improve generally thesimplicity and eliiciency of automatic frequency control circuits forradio receivers, and more especially to provide AFC circuits which arereliable in operation andY economical of construction.

The novel features which I believe to be characteristic of my inventionare set forth in paricularity in the appended claims the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I haveindicated'diagrammatically a circuit organization whereby my inventionmay be carried into effect.

In the drawing:

Fig, 1 shows a superheterodyne receiver embodyng one form of theinvention,

Fig. 2 graphically demonstrates the functioning of the AFCdiscriminator.

Referring now to Fig. l, there is shown a super-l heterodyne receivercircuit embodying an AFC circuit constructed according to lmy invention.The receiver generally comprises a signal collector l the latter may begrounded antenna, a loop collector, a radio frequency distribution lineor the Vcollector` of a mobile structure such as an automobile. Thesignals collected by the collector l are impressed upon the tunablesignal input circuit of the rst transmission network.A The latter may bea radio frequency amplifier followed by one or more tunable radiofrequency amplifiers. For the sake of simplicity, the first network hasbeen shown as the rst detector 2; the tunable input circuit 3 thereof iscoupled to collector l. The signals may be chosen from the broadcastband of 500 to 1500 kc.; if desired, the receiver can be of themulti-range type adapted to receive short Wave signals as well. Thelocal oscillator ll may bey of any desired type; it has a tunablev tankcircuit comprising the variable tuning condenser 5 and coil 6. TheoscillatorA is tunable over a frequency range which differs fromthesignal frequency range by the value of the operating I. F., the lattermay be chosen from a range of 75 to 450 kc. The rotors of the signalcircuit variable condensers are arranged for mechanical uni-control withthe oscillator condenser rotors; the dotted line l depicts such tuningmechanism. The local oscillations produced by oscillator .are impressedupon the first detector circuit; the LF.; energy is developed in theresonant circuit .8 which is tuned to the I. F.

The I. F, amplifier 9 is provided with an I. F.- tuned input circuit l0,the latter being coupled to circuit 8. The I. F.tuned output circuit l lof the amplifier is magnetically coupled, as at M, to the circuit l2which is resonated to the operating I. F. value. j The high potentialside of circuit ll vis coupled to the midpoint lli of the coil l2'l ofthe secondary circuit l2 by the condenser l having av magnitude ofapproximately 50 mmf. The point 'l5 of circuit I 2 is connected to theanode ll of the tube 55 by condenser it. The opposite point I6 ofcircuit I2 is connected to the anode I9 of tube 55 by condenser 2li. Thecondensersl and 2li may have a value of 50 to 500 mmf. The tube 55 is ofthe duo-diode triode type; the common cathode` 2l` is connected toground through` a bias resistor y22, thelatter ;be-.:y

ing shunted by an I F. bypass condenser 23. The diode load resistors forboth diodes are 2li and 25; the junction of the resistors is connectedto cathode 2| and the resistor 22. The resistors may each have a valueof 500,000 ohms.

'Ihe midpoint I4 of the coil |2' is connected to ground through aresistor 26 of about 1 megohm in value; the plate 21 of tube 55cooperates with cathode 2| and grid 26 to provide an audio ampliersection. The grid 26 is connected to resistor 26 so that grid 23 is at aproper negative bias for audio amplification; and, also, to derive anaudio voltage therefrom. The plate 21 may be connected to a desiredpositive voltage source by connecting it to such source through resistor30. The amplified audio voltage is transmitted through couplingcondenser 3| to a following audio network.

The frequency control network comprises a pair of electron dischargetube sections; in Fig. l these sections are shown disposed in a commontube envelope 32.` The latter includes one triode having a cathode 33, agrid 35 and plate 35; and a second triode having a cathode 35, a grid 31and a plate 38. The two cathodes are connected in common to groundthrough a biasing resistor-condenser network 33. Plate 35 is connectedto one end of coil 40; the opposite end is connected to plate 38. Themidpoint of coil i0 is connected to an energizing potential source forthe plates 35 and 38. coupled to tank coil 5.

There is connected in shunt across the tank circuit 5 6, a pathincluding resistor 4| and condenser 42 in series. The radio frequencyvoltage developed across condenser l2 is applied in parallel to grids 34and 3l' through condensers 43 and 44. The tube 32 and its connections tothe tank circuit 5 6 provide a quadrature type frequency shift circuit.The plates of tube 32 are in push-pull relation to the oscillator tankcircuit; the control grids are in parallel relation to the latter. Ofcourse, the reverse relations may be used, as disclosed in my co-pendingapplication Serial No. 130,630, led March 13, 1937. The specific controltube arrangement shown in Fig. 1 is that disclosed in Fig. 2 of my saidcopending application.

The control grids of 'tube 32 are connected to the discriminator diodes;Thus, grid 33 is connected by lead 45 to the anode end of resistor 24.Grid 31 is connected by lead l5 to the anode end of resistor 25. Thedirect current Voltage developed across resistor 2K3 is filtered byseries resistor 41 and shunt condenser 58. Resistor 43 acts as acoupling element between resistor 22 and grid 34. Resistor 50 andcondenser 5| function as a filter network for the direct current voltagedeveloped across resistor 25; resistor 52 acts as the coupling elementto grid 31'. The cathode 2| is connected to the junction of condensers48 and 5|.

The leads A-46 have been designated as the AFC connections. It will beunderstood, particularly by reference to Fig. 2, that the AFC leadsapply only negative voltage to grids 35 and 31. From a generic View, theaction of tube 32 is to produce, or simulate, across tank circuit 5 5positive and negative inductive reactances. Although the grids 34 and 31are at a common radio frequency potential, due to the push-pull couplingof plates 35-33 to the tank circuit 5 6 the inductive reactancessimulated across the tank circuit will be of different sign. When thegrids .34 and 31 are at the same direct current Coil |13 is magneticallypotential, and this occurs at a value fn equal to the operating I. F.,then no frequency shift occurs in tank circuit 5 6. A change infrequency value of the I. F. energy results in an unbalance of the D. C.biases of the grids 34 and 31; since these biases are different the A.C. plate currents of tube 32 will be unequal and a frequency shift inthe tank circuit results. The direction of the frequency shift of thetank circuit depends upon the polarity of coils 6 and 40. Reversing thetwo ends of coil 43 will reverse the direction of frequency shift.

Considering, now, the discriminator tube 55, it will be observed thatthe cathode 2| is connected to ground through the bias resistor 22. Thispermits the use of a common cathode tube, instead of using a pair ofindependent diodes as in my aforesaid copending application. A distinctadvantage in this type of circuit resides in the fact that aduo-diode-triode, or 55 type tube, may be used, and the triode sectionis employed as an audio amplifier. Prior discriminator circuits of thisphase shift type have not permitted the use of a common grounded cathodefor the diode rectiiiers; the present circuit read.- ily admits of thelatter.

The functioning of the phase shift discriminator circuits |2 have beenfully described by S. Seeley in application Serial No. 45,413, filedOct. 11, 1935, Patent No. 2,121,103 issued June 21, 1938; it is notthought necessary to explain the circuit in detail. It is believedsufcient to point out that the Diode curves in Fig. 2 show the change inD. C. voltage produced across each diode load resistor as the I. F.energy departs in frequency from the operating value To. Each dioderectifier develops a D. C. Voltage which is negative with respect toground, but which is a maximum on one side or the other of the I. F.carrier value fn. The audio voltage (marked Audio in Fig. 2) is tappedonc from resistor 25'. In general, the audio voltage will, also, showtwo peaks on either side of the I. F. carrier value. Each of the lternetworks 111-28 and 53 5| acts to remove the I. F. carrier componentfrom the negative D. C. voltage applied over leads i5-56 to the controlgrids of tube 32.

In order to obtain AVC voltage Which is symmetrical with respect to thefrequency fo, resistors 50 and 10 are connected as shown to the AVClead. If these resistors are high in resistance compared to 23 and 25,the AVC voltage will be the average potential of that of the two diodes.This means that it will vary with frequency (I. F. value) in the mannerillustrated in Fig. 2. Of course, the AVC lead 80 will be connected toas many of the pre-detector tubes as may be desired. Suitable I. F.lters may be included in lead 80.

In actual operation, when the frequency of the resultant I. F. energydiffers from that to which circuits |2 are resonated, the D. C. voltageacross one of the resistors 24, 25 is higher than that across the other.rIhis produces an unbalance in the grids of tube 32, as regards negativepotential; asa consequence of this unbalance there results a change infrequency in the direction to bring the I. F. energy back to normalfrequency. The polarity of coil 12', relative to the coil determineswhich resistor, 24 or 25, will have to develop the greater D. C. voltagewhen the frequency of the I. F. energy is higher than the normal, oroperating, I. F. value. Similarly, the polarity of coil 40 determineswhether the oscillation frequency will increase or decrease when grid 34is more negative than grid 31.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scopey of my invention, as set forth in the appended claims.

What I claim is:

l. In a detection network, a primary circuit, a secondary circuitcoupled thereto, the twocircuits each being tuned to a commonfrequency,means establishing the midpointof the secondary circuit at the potentialof one end of the primary circuit, a pair of diode rectiers havingcathodes at a common potential, a load resistor connected between eachdiode anode and cathode, and means for coupling the diode anodes toopposite ends of the secondary circuit.

2. In a detection network, a primary circuit, a secondary circuitcoupled thereto, the two circuits each being tuned to a commonfrequency, means establishing the midpoint of the secondary v circuit atYthe potential of one end of the primary circuit, a pair of dioderectiers having cathodes at a common potential, a load resistorconnected between each diode anode and cathode, means for coupling thediode anodes to opposite ends of the secondary circuit, and a directcurrent voltage connection to the anode side of each load resistor.

3. In a detection network, a primary circuit, a secondary circuitcoupled thereto, the two circuits each being tuned to a commonfrequency, means establishing the midpoint of the secondary circuit atthe potential of one end of the primary circuit, a pair of dioderectiers having cathodes at a common potential, a load resistorconnected between each diodie anode and cathode, and means for couplingthe diode `anodes to opposite ends of the secondary circuit, saidcathodes being a common cathode electrode, a common tube envelopehousing the latter and the anodes of the rectiers.

4. In a detection network, a primary circuit, a secondary circuitcoupled thereto, the two circuits each being tuned to a commonfrequency, means establishing the midpoint of the secondary circuit atthe potential of one end of the primary circuit, a pair of dioderectiilers having cathodes at a common potential, a load resistorconnected between each diode anode and cathode,'means for coupling thediode anodes to opposite ends of the secondary circuit, and meansestablishing4 said midpoint at saidcommon potential.

5. In a receiver, an automatic frequency control system which includes adiscriminator network, said network comprising a pair of diode rectiersof the type including a single tube housing a common cathode and a pairof associated anodes, a signal input circuit connected to said anodes, aload resistance path connected between said anodes, means connecting thecathode to the midpoint of said resistance path, said midpoint beingestablished at ground potential, and means for establishing the midpointof said input circuit at ground potential. Y

6. In a superheterodyne receiver of the type including a frequencychanger network, a local oscillator provided with a resonant tankcircuit, an

7. In a superheterodyne receiver of the type u including a localoscillator having a tank circuit and a first detector circuit having anintermediate frequency output circuit; the improvement which comprisesan electron discharge tube having at least a cathode and two anodes tolprovide a pair of diodes, a resonant circuit coupled to said outputcircuit and tuned to the same frequency as the latter, meansestablishing the midpoint of the resonant circuit at the alternatingpotential of the high potential side of said output circuit, meansconnecting said cathode to said midpoint, means connecting said anodesto opposite sides of the said resonant circuit, a resistive pathconnected between said anodes and having the midpoint thereof connectedto said cathode, a frequency control tube comprising a pair of electro-ndischarge devices having a common output circuit coupled to said tankcircuit, each device having an input electrode, means impressing analternating potential derived from the tank circuit upon said inputelectrodes, and a direct current voltage connection between each of saidinput electrodes and its respective anode end of said resistive path.

8. In a superheterodyne receiver of the type including a localoscillator having a tank circuit and a first detector circuit having anintermediate frequency output circuit; thev improvement which comprisesan electron discharge tube having at least a cathode and two anodes toprovide a pair of diodes, a resonant circuit coupled to said outputcircuit and tuned to the same frequency as the latter, meansestablishing the midpoint of the resonant circuit at the alternatingpotential of the high potential side of said output circuit, meansconnecting said cathode to said midpoint, means connecting said anodesto opposite sides of the said resonant circuit, a resistive pathconnected between said anodes and having the midpoint thereof connectedto said cathode, a frequency control tube comp-rising a pair of electrondischarge devices having a common output circuit coupled to said tankcircuit, each device having an input electrode, means impressing analternating potential derived from the tank circuit upon said inputelectrodes, and a direct current voltage connection between each of saidinput electrodes and its respective anode end of said resistive path, anaudio utilization network coupled to said resistive path to derivetherefrom the audio voltage developed thereacross,` and an automaticvolume control circuit connected to said resistive path to derivetherefrom a direct current voltage whose amplitude is proportional tothe amplitude of the intermediate frequency energy.

DUDLEY E. FOSTER.

